Image display mirror for a vehicle

ABSTRACT

An image display mirror for a vehicle includes a first polarizing plate arranged so as to be rotatable in a plane, a half mirror having a reflection axis and configured to reflect polarized light, and an image display apparatus in the stated order from a viewer side, in which: the first polarizing plate has a polarizer; a rotation of the first polarizing plate changes an angle of an absorption axis of the polarizer relative to a reflection axis of the half mirror when an image is not displayed on the image display apparatus and when the image is displayed thereon; when the image is not displayed, the absorption axis of the polarizer and the reflection axis of the half mirror are perpendicular to each other; and when the image is displayed, the absorption axis of the polarizer and the reflection axis of the half mirror are parallel to each other.

This application claims priority under 35 U.S.C. Section 119 to JapanesePatent Application No. 2015-018656 filed on Feb. 2, 2015, which isherein incorporated by references.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image display mirror for a vehicle.

2. Description of the Related Art

A technology involving combining a rear-view mirror for a vehicle withan image display apparatus to display an image has heretofore beenknown. For example, Japanese Patent No. 5273286 discloses an imagedisplay mirror including a half mirror arranged on the front surface(viewer side surface) of a monitor. In the image display mirror, therear can be viewed with a reflected image provided by the half mirror.Meanwhile, when an image is displayed on the monitor, the image can beviewed through the half mirror.

Such image display mirror involves a problem in that, for example, whenthe quantity of light from the rear of a vehicle is large, the reflectedimage inhibits the visibility of an image displayed on the monitor.Japanese Patent No. 5273286 proposes the following technology. Aninfluence of the reflected image is reduced by making the angle of thehalf mirror when a viewer (occupant) views the rear and the angle whenthe viewer views the image of the monitor different from each other.According to such technology, the influence of the reflected imageprovided by the half mirror can be reduced by adjusting the angle of thehalf mirror so that when the monitor image is viewed, the reflectedimage becomes an image that does not inhibit the visibility of themonitor image, specifically so that a ceiling is mirrored by reflection.

However, when it is difficult to turn the reflected image provided bythe half mirror into the image that does not inhibit the visibility ofthe monitor image, e.g., when the image display mirror of JapanesePatent No. 5273286 is applied to a vehicle including a ceiling thattransmits light, such as a panoramic roof or a sunroof, or a convertiblecar, the influence of the reflected image cannot be reduced by themirror.

SUMMARY OF THE INVENTION

The present invention has been made to solve the conventional problems,and an object of the present invention is to provide an image displaymirror that includes a half mirror and an image display apparatus,reduces an influence of a reflected image provided by the half mirror,and is excellent in visibility of an image displayed on the imagedisplay apparatus.

An image display mirror for a vehicle according to one embodiment of thepresent invention includes a first polarizing plate arranged so as to berotatable in a plane, a half mirror having a reflection axis and beingconfigured to reflect polarized light, and an image display apparatus inthe stated order from a viewer side, in which: the first polarizingplate has a polarizer; a rotation of the first polarizing plate changesan angle of an absorption axis of the polarizer relative to a reflectionaxis of the half mirror when an image is not displayed on the imagedisplay apparatus and when the image is displayed thereon; when theimage is not displayed, the absorption axis of the polarizer and thereflection axis of the half mirror are perpendicular to each other; andwhen the image is displayed, the absorption axis of the polarizer andthe reflection axis of the half mirror are parallel to each other.

In one embodiment of the present invention, the first polarizing plateis subjected to a low-reflection treatment.

In one embodiment of the present invention, the image display mirror fora vehicle further includes a λ/4 plate on a viewer side of the firstpolarizing plate.

In one embodiment of the present invention, the image display apparatusincludes a liquid crystal display apparatus including a liquid crystalcell, and the liquid crystal display apparatus is free of a polarizingplate on a viewer side of the liquid crystal cell.

According to another embodiment of the present invention, there isprovided a method of observing surroundings of a vehicle. The method isa method by which a driver of a vehicle observes surroundings of thevehicle with the above-mentioned image display mirror for a vehicle, themethod including: switching attached and removed states of the firstpolarizing plate when an image is displayed on the image displayapparatus and when the image is not displayed thereon; and arranging,when the image is displayed, the first polarizing plate between the halfmirror and the driver of the vehicle.

The image display mirror for a vehicle according to the embodiment ofthe present invention includes the polarizing plate arranged so as to berotatable in a plane, the half mirror configured to reflect polarizedlight, and the image display apparatus in the stated order from theviewer side. In such image display mirror for a vehicle, an influence ofa reflected image provided by the half mirror is reduced, and hence thevisibility of an image displayed on the image display apparatus isexcellent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an image display mirroraccording to one embodiment of the present invention.

FIG. 2A and FIG. 2B are each a schematic view for illustrating an actionaccording to the one embodiment of the present invention.

FIG. 3 is a schematic perspective view of an example of a reflectivepolarizer to be used in the one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are hereinafter described withreference to the drawings. However, the present invention is not limitedto these embodiments.

A. Overall Construction of Image Display Mirror for Vehicle

FIG. 1 is a schematic sectional view of an image display mirroraccording to one embodiment of the present invention. An image displaymirror 100 for a vehicle includes a first polarizing plate 110 arrangedso as to be rotatable in a plane, a half mirror 120, and an imagedisplay apparatus 130 in the stated order from a viewer side. The halfmirror 120 and the image display apparatus 130 are preferably arrangedso as to be parallel to each other. The image display mirror for avehicle of this embodiment can be used as, for example, the rear-viewmirror (room mirror) of a vehicle. The half mirror 120 has alight-reflecting function and a light-transmitting function. The imagedisplay mirror 100 for a vehicle enables an occupant (more specifically,a driver) of the vehicle to observe the surroundings (e.g., the rear) ofthe vehicle by virtue of the light-reflecting function of the halfmirror 120. In addition, in the image display mirror 100 for a vehicle,an image displayed on the image display apparatus 130 can be viewed byvirtue of the light-transmitting function of the half mirror 120. Theimage display apparatus 130 displays, for example, an image provided byan external camera that mirrors the surroundings (e.g., the rear) of thevehicle. With such construction, even, for example, when an obstacle(such as a passenger or baggage) is present in the vehicle and hence thesurroundings of the vehicle cannot be sufficiently observed with thereflected image of the half mirror, the safety of the vehicle can besecured by displaying the image provided by the external camera on theimage display apparatus. It should be noted that, although notillustrated, the image display mirror for a vehicle of the presentinvention may further include any appropriate other member.

The half mirror is a half mirror that can reflect polarized light. Morespecifically, the half mirror has a reflection axis and a transmissionaxis perpendicular to each other, and can reflect polarized light whosepolarization direction is parallel to the reflection axis and transmitpolarized light whose polarization direction is parallel to thetransmission axis.

As described above, the image display mirror for a vehicle of thepresent invention includes the first polarizing plate arranged so as tobe rotatable in a plane. More specifically, in the image display mirrorfor a vehicle of the present invention, the rotation of the firstpolarizing plate (rotation by preferably from 85° to 95°, morepreferably from 88° to 92°, still more preferably 90°) changes the angleof the absorption axis of the polarizer of the first polarizing platerelative to the reflection axis of the half mirror when the image is notdisplayed on the image display apparatus and when the image is displayedthereon. When the image is not displayed, the absorption axis of thepolarizer and the reflection axis of the half mirror are perpendicularto each other. Meanwhile, when the image is displayed, the absorptionaxis of the polarizer and the reflection axis of the half mirror areparallel to each other. It should be noted that the term “rotation in aplane” as used herein does not mean a strict rotation in the plane, butmeans such a rotation that the angle between the absorption axis of thepolarizer and the reflection axis of the half mirror when viewed fromthe viewer side can change from a perpendicular angle to a parallelangle or from the parallel angle to the perpendicular angle to theextent that a viewer can view the half mirror through the firstpolarizing plate. In addition, the term “parallel” as used hereinincludes the case where the axes are substantially parallel to eachother. Here, the phrase “substantially parallel” includes the case wherean angle between the axes is 0°±110°, and the angle is preferably 0°±7°,more preferably 0°±5°. Further, the term “perpendicular” as used hereinincludes the case where the axes are substantially perpendicular to eachother. Here, the phrase “substantially perpendicular” includes the casewhere the angle between the axes is 90°±10°, and the angle is preferably90°±7°, more preferably 90°±5°.

FIG. 2A and FIG. 2B are each a schematic view for illustrating an actionaccording to the one embodiment of the present invention. In FIG. 2A, astate in which the image is not displayed on the image display apparatus130 and the reflected image provided by the half mirror 120 is subjectedto viewing is illustrated. In this state, the first polarizing plate 110is arranged so that an absorption axis C of the polarizer of the firstpolarizing plate 110 and a reflection axis A of the half mirror 120 areperpendicular to each other. Accordingly, polarized light generated bytransmission through the first polarizing plate 110 from the viewer sidebecomes polarized light whose polarization direction is parallel to thereflection axis A, and is reflected by the half mirror 120. In addition,the reflected light is transmitted through the first polarizing plate110. As a result, the occupant of the vehicle can view the reflectedimage provided by the half mirror 120. In FIG. 2B, a state in which theimage is displayed on the image display apparatus 130 is illustrated,and the first polarizing plate rotates by about 90° from the state ofFIG. 2A. In this state, the first polarizing plate 110 is arranged sothat the absorption axis C of the polarizer of the first polarizingplate 110 and the reflection axis A of the half mirror 120 are parallelto each other. Accordingly, the polarized light generated by thetransmission through the first polarizing plate 110 from the viewer sidebecomes polarized light whose polarization direction is perpendicular tothe reflection axis A (i.e., parallel to a transmission axis B), and istransmitted through the half mirror 120. As a result, the reflectedimage provided by the half mirror 120 becomes difficult to view. On theother hand, the light transmitted through the half mirror 120 from theimage display apparatus 130 is polarized light that can be transmittedthrough the first polarizing plate 110, and hence the image of the imagedisplay apparatus 130 is subjected to the viewing. As described above,according to the image display mirror for a vehicle of the presentinvention, the influence of the reflected image provided by the halfmirror 120 is reduced and hence the visibility of the image displayed onthe image display apparatus 130 can be improved.

Any appropriate mechanism can be adopted as a mechanism for rotating thefirst polarizing plate as long as the effects of the present inventionare obtained. For example, such a mechanism that the first polarizingplate is held with a frame body and the first polarizing plate isrotated in the frame body (preferably rotated by 90° as one unit) isgiven.

The half mirror and the image display apparatus may be brought intocontact with each other or may be out of contact with each other. It ispreferred that a gap between the half mirror and the image displayapparatus be filled with a transparent resin, and both the members bebrought into close contact with each other. When both the members arebrought into close contact with each other as described above, an imagedisplay mirror for a vehicle excellent in efficiency with which light isutilized and excellent in visibility of a displayed image can beobtained. Any appropriate resin film, pressure-sensitive adhesive, orthe like can be used in interlayer filling. A pressure-sensitiveadhesive excellent in transparency is preferably used as thepressure-sensitive adhesive. Examples thereof include an acrylicpressure-sensitive adhesive, a silicone-based pressure-sensitiveadhesive, and a rubber-based pressure-sensitive adhesive.

In one embodiment, a λ/4 plate can be arranged on the viewer side (i.e.,the side opposite to the half mirror) of the first polarizing plate. Theλ/4 plate has a function of transforming linearly polarized light intocircularly polarized light (or circularly polarized light into linearlypolarized light) by arranging its slow axis at an angle of about +45° orabout −45° relative to the absorption axis of the first polarizing plate(details are described later). The arrangement of the λ/4 plate canprovide an image display mirror for a vehicle excellent in visibilityfor a user of a pair of polarized sunglasses. It should be noted thatthe λ/4 plate may be brought into contact with the first polarizingplate or may be out of contact therewith. In addition, the λ/4 plate andthe first polarizing plate may be bonded to each other throughintermediation of a pressure-sensitive adhesive layer. Further, the λ/4plate may be arranged removably and attachably.

B. First Polarizing Plate

The first polarizing plate typically has a polarizer and a protectivelayer arranged on one side, or each of both sides, of the polarizer. Thepolarizer is typically an absorption-type polarizer.

The transmittance (also referred to as “single axis transmittance”) ofthe polarizer at a wavelength of 589 nm is preferably 41% or more, morepreferably 42% or more. It should be noted that a theoretical upperlimit for the single axis transmittance is 50%. In addition, itspolarization degree is preferably from 99.5% to 100%, more preferablyfrom 99.9% to 100%.

Any appropriate polarizer may be used as the polarizer. Examples thereofinclude: a polarizer obtained by adsorbing a dichroic substance, such asiodine or a dichroic dye, onto a hydrophilic polymer film, such as apolyvinyl alcohol-based film, a partially formalized polyvinylalcohol-based film, or an ethylene-vinyl acetate copolymer-basedpartially saponified film, and subjecting the resultant film to uniaxialstretching; and polyene-based alignment films, such as a dehydratedproduct of polyvinyl alcohol and a dehydrochlorinated product ofpolyvinyl chloride. Of those, a polarizer obtained by adsorbing adichroic substance, such as iodine, onto a polyvinyl alcohol-based filmand subjecting the resultant film to uniaxial stretching is particularlypreferred because of its high polarized dichromaticity. The polarizerhas a thickness of preferably from 0.5 μm to 80 μm.

The polarizer obtained by adsorbing iodine onto a polyvinylalcohol-based film and subjecting the resultant film to uniaxialstretching is typically produced by dyeing polyvinyl alcohol throughimmersion in an aqueous solution of iodine and stretching the resultantfilm at a ratio of from 3 times to 7 times with respect to its originallength. The stretching may be carried out after the dyeing, thestretching may be carried out during the dyeing, or the stretching maybe carried out before the dyeing. The polarizer may be produced bysubjecting the film to treatments such as swelling, cross-linking,adjusting, washing with water, and drying in addition to the stretchingand the dyeing.

Any appropriate film may be used as the protective layer. As a materialfor the main component of such film, there are specifically given, forexample: cellulose-based resins, such as triacetylcellulose (TAC); andtransparent resins, such as (meth)acrylic, polyester-based, polyvinylalcohol-based, polycarbonate-based, polyamide-based, polyimide-based,polyether sulfone-based, polysulfone-based, polystyrene-based,polynorbornene-based, polyolefin-based, or acetate-based transparentresins. In addition, examples thereof further include thermosettingresins and UV curable resins, such as acrylic, urethane-based, acrylicurethane-based, epoxy-based, or silicone-based thermosetting resins andUV curable resins. In addition, examples thereof further include glassypolymers, such as a siloxane-based polymer. In addition, a polymer filmdescribed in Japanese Patent Application Laid-open No. 2001-343529(International Patent WO01/37007A) may also be used. For example, aresin composition containing a thermoplastic resin having in its sidechain a substituted or unsubstituted imide group and a thermoplasticresin having in its side chain a substituted or unsubstituted phenylgroup and a nitrile group may be used as a material for the film. Anexample thereof is a resin composition containing an alternatingcopolymer formed of isobutene and N-methylmaleimide and anacrylonitrile-styrene copolymer. The polymer film may be, for example,an extruded product of the resin composition.

In one embodiment, the first polarizing plate is subjected to alow-reflection treatment. The surface of the protective layer ispreferably subjected to the low-reflection treatment. The low-reflectiontreatment is, for example, a treatment involving forming a layer, suchas a fluorine-based resin layer, a multilayer metal-deposited layer, anoptical interference layer, or a layer having a fine uneven shape (e.g.,a moth-eye structure).

C. Half Mirror

A reflective polarizing plate including a reflective polarizer is usedas the half mirror. The reflective polarizer has functions oftransmitting polarized light in a specific polarization state(polarization direction) and reflecting light in a polarization stateexcept the foregoing. The reflective polarizer is preferably of alinearly polarized light separation type.

FIG. 3 is a schematic perspective view of an example of the reflectivepolarizer. The reflective polarizer is a multilayer laminate in which alayer A having birefringence and a layer B substantially free ofbirefringence are alternately laminated. The total number of the layersof such multilayer laminate can be, for example, from 50 to 1,000. Inthe illustrated example, a refractive index nx of the layer A in anx-axis direction is larger than a refractive index ny thereof in ay-axis direction, and a refractive index nx of the layer B in the x-axisdirection and a refractive index ny thereof in the y-axis direction aresubstantially equal to each other. Therefore, a refractive indexdifference between the layer A and the layer B is large in the x-axisdirection, and is substantially zero in the y-axis direction. As aresult, the x-axis direction serves as a reflection axis and the y-axisdirection serves as a transmission axis. The refractive index differencebetween the layer A and the layer B in the x-axis direction ispreferably from 0.2 to 0.3. It should be noted that the x-axis directioncorresponds to the stretching direction of the reflective polarizer.

The layer A is preferably constituted of a material that expressesbirefringence when stretched. Typical examples of such material includenaphthalene dicarboxylic acid polyester (such as polyethylenenaphthalate), polycarbonate, and an acrylic resin (such as polymethylmethacrylate). Of those, polyethylene naphthalate is preferred. Thelayer B is preferably constituted of a material that is substantiallyfree from expressing birefringence even when stretched. A typicalexample of such material is the copolyester of naphthalene dicarboxylicacid and terephthalic acid.

The reflective polarizer transmits light having a first polarizationdirection (such as a p-wave) at an interface between the layer A and thelayer B, and reflects light having a second polarization directionperpendicular to the first polarization direction (such as an s-wave) atthe interface. Part of the reflected light is transmitted as lighthaving the first polarization direction, and other part thereof isreflected as light having the second polarization direction at theinterface between the layer A and the layer B.

The total thickness of the reflective polarizer can be appropriately setin accordance with, for example, purposes and the total number of thelayers in the reflective polarizer. The total thickness of thereflective polarizer is preferably from 10 μm to 150 μm.

The reflective polarizer can be typically produced by combiningco-extrusion and lateral stretching. The co-extrusion can be performedby any appropriate system. For example, a feed block system ispermitted, or a multi-manifold system is permitted. For example, thematerial for constituting the layer A and the material for constitutingthe layer B are extruded in a feed block, and then the resultant isturned into a plurality of layers with a multiplier. It should be notedthat such apparatus for turning the materials into the plurality oflayers is known to a person skilled in the art. Next, the resultantelongated multilayer laminate is typically stretched in a direction (TD)perpendicular to a conveying direction. The material for constitutingthe layer A (such as polyethylene naphthalate) is increased inrefractive index only in the stretching direction by the lateralstretching, and as a result, expresses birefringence. The material forconstituting the layer B (such as the copolyester of naphthalenedicarboxylic acid and terephthalic acid) is not increased in refractiveindex in any direction even by the lateral stretching. As a result, areflective polarizer having a reflection axis in the stretchingdirection (TD) and having a transmission axis in the conveying direction(MD) can be obtained (the TD corresponds to the x-axis direction of FIG.3 and the MD corresponds to the y-axis direction thereof). It should benoted that a stretching operation can be performed with any appropriateapparatus.

Another example of the linearly polarized light separation-typereflective polarizer is such a polarizing fiber or polarizing wovenfabric as described in Japanese Patent Application Laid-open No.2009-24318. The performance of the reflective polarizer improves as arefractive index difference in a direction perpendicular to thelengthwise direction of the polarizing fiber becomes smaller and arefractive index difference in the lengthwise direction of thepolarizing fiber becomes larger. Still another example of the linearlypolarized light separation-type reflective polarizer is such a wire gridpolarizer as described in Japanese Patent Application Laid-open No.2011-48630.

A commercial product may be directly used as the reflective polarizer,or the commercial product may be subjected to secondary processing (suchas stretching) before use. Examples of the commercial product include aproduct available under the trade name “DBEF” from 3M Company and aproduct available under the trade name “APF” from 3M Company. Inaddition, the wire grid polarizer is, for example, a product availableunder the trade name “WGFTM” from Asahi Kasei E-materials.

D. Image Display Apparatus

Any appropriate apparatus can be used as the image display apparatus.Examples thereof include a liquid crystal display apparatus, an organicEL display apparatus, and a plasma display apparatus. Description isgiven below by taking the liquid crystal display apparatus as a typicalexample. In one embodiment, as the liquid crystal display apparatus,there is used an image display apparatus including, as illustrated inFIG. 1, a liquid crystal panel including a liquid crystal cell 131, asecond polarizing plate 132 arranged on the viewer side of the liquidcrystal cell 131, and a third polarizing plate 133 arranged on the backsurface side of the liquid crystal cell 131. It should be noted that,although not illustrated, the image display apparatus may include anyappropriate other member (such as a backlight unit) as required.

D-1. Liquid Crystal Cell

The liquid crystal cell 131 has a pair of substrates and a liquidcrystal layer serving as a display medium sandwiched between thesubstrates. In a general construction, a color filter and a black matrixare arranged on one of the substrates, and a switching element forcontrolling the electrooptical characteristics of liquid crystal, ascanning line for providing the switching element with a gate signal anda signal line for providing the element with a source signal, and apixel electrode and a counter electrode are arranged on the othersubstrate. An interval between the substrates (cell gap) can becontrolled with, for example, a spacer. For example, an alignment filmformed of polyimide can be arranged on the side of each of thesubstrates to be brought into contact with the liquid crystal layer.

In one embodiment, the liquid crystal layer contains liquid crystalmolecules aligned in a homogeneous array under a state in which noelectric field is present. Such liquid crystal layer (resultantly theliquid crystal cell) typically shows a three-dimensional refractiveindex of nx>ny=nz. It should be noted that the expression “ny=nz” asused herein includes not only the case where ny and nz are completelyequal to each other but also the case where ny and nz are substantiallyequal to each other. Typical examples of a driving mode using the liquidcrystal layer showing such three-dimensional refractive index include anin-plane switching (IPS) mode and a fringe field switching (FFS) mode.It should be noted that the IPS mode includes a super in-plane switching(S-IPS) mode and an advanced super in-plane switching (AS-IPS) mode eachadopting a V-shaped electrode, a zigzag electrode, or the like. Inaddition, the FFS mode includes an advanced fringe field switching(A-FFS) mode and an ultra fringe field switching (U-FFS) mode eachadopting a V-shaped electrode, a zigzag electrode, or the like.

In another embodiment, the liquid crystal layer contains liquid crystalmolecules aligned in a homeotropic array under a state in which noelectric field is present. Such liquid crystal layer (resultantly theliquid crystal cell) typically shows a three-dimensional refractiveindex of nz>nx=ny. A driving mode using the liquid crystal moleculesaligned in the homeotropic array under a state in which no electricfield is present is, for example, a vertical alignment (VA) mode. The VAmode includes a multi-domain VA (MVA) mode.

D-2. Second Polarizing Plate and Third Polarizing Plate

Such polarizing plate as described in the section B is used as each ofthe second polarizing plate and the third polarizing plate.

The second polarizing plate and the third polarizing plate can bearranged so that the absorption axes of their respective polarizers aresubstantially perpendicular or parallel to each other to enable theviewing of an image. In addition, the second polarizing plate ispreferably arranged so that the absorption axis of the second polarizingplate and the reflection axis of the half mirror are parallel to eachother.

In one embodiment, the second polarizing plate 132 is omitted from theimage display apparatus (liquid crystal display apparatus) 130illustrated in FIG. 1. That is, in this embodiment, a liquid crystaldisplay apparatus free of a polarizing plate on the viewer side of itsliquid crystal cell is used. In this embodiment, the brightness of theimage display mirror for a vehicle can be improved because an opticalloss due to the second polarizing plate can be eliminated. In thisembodiment, when the first polarizing plate is brought into the attachedstate, the first polarizing plate and the third polarizing plate arearranged so that the absorption axes of their respective polarizers aresubstantially perpendicular or parallel to each other to enable theviewing of an image. In addition, in this embodiment, the liquid crystaldisplay apparatus can be configured so that polarized light output fromthe liquid crystal display apparatus can be transmitted through the halfmirror.

E. λ/4 Plate

In one embodiment , as described above, the λ/4 plate is arranged on theviewer side (i.e., the side opposite to the half mirror) of the firstpolarizing plate.

A front retardation R₀ of the λ/4 plate at a wavelength of 590 nm isfrom 90 nm to 190 nm, preferably from 100 nm to 180 nm, more preferablyfrom 110 nm to 170 nm. It should be noted that the front retardation R₀in this specification is determined from the equation “R₀=(nx−ny)×d”where nx represents a refractive index in the direction in which anin-plane refractive index becomes maximum (i.e., a slow axis direction),ny represents a refractive index in a direction perpendicular to theslow axis in a plane (i.e., a fast axis direction), and d (nm)represents the thickness of a retardation film, the parameters beingvalues under 23° C. The λ/4 plate shows any appropriate refractive indexellipsoid as long as the plate has the relationship of nx>ny. Forexample, the refractive index ellipsoid of the λ/4 plate shows therelationship of nx>nz>ny or nx>ny≧nz.

An angle between the absorption axis of the polarizer of the firstpolarizing plate and the slow axis of the λ/4 plate is preferably from+40° to +50° or from −40° to −50°, more preferably from +43° to +47° orfrom −43° to −47°, still more preferably +45° or −45°. When the firstpolarizing plate and the λ/4 plate are arranged so as to show suchrelationship, the laminated structure of the first polarizing plate andthe λ/4 plate can function as a circularly polarizing plate.

Any appropriate material can be used as a material for constituting theλ/4 plate as long as the effects of the present invention are obtained.A typical example thereof is a stretched film of a polymer film.Examples of a resin for forming the polymer film include apolycarbonate-based resin and a cycloolef in-based resin. A method ofproducing the λ/4 plate is not particularly limited, but the λ/4 platecan be obtained by, for example, stretching the polymer film at atemperature of from about 100° C. to about 250° C. and at a stretchingratio of from about 1.1 times to about 2.5 times. The front retardationand thickness direction retardation of the λ/4 plate can be controlledby adjusting the stretching ratio and stretching temperature of thepolymer film. The thickness and total light transmittance of the λ/4plate are preferably about 200 μm or less and 80% or more, respectively,though the thickness and the total light transmittance are notparticularly limited thereto.

What is claimed is:
 1. An image display mirror for a vehicle, comprisinga first polarizing plate arranged so as to be rotatable in a plane, ahalf mirror having a reflection axis and being configured to reflectpolarized light, and an image display apparatus in the stated order froma viewer side, wherein: the first polarizing plate has a polarizer; arotation of the first polarizing plate changes an angle of an absorptionaxis of the polarizer relative to a reflection axis of the half mirrorwhen an image is not displayed on the image display apparatus and whenthe image is displayed thereon; when the image is not displayed, theabsorption axis of the polarizer and the reflection axis of the halfmirror are perpendicular to each other; and when the image is displayed,the absorption axis of the polarizer and the reflection axis of the halfmirror are parallel to each other.
 2. The image display mirror for avehicle according to claim 1, wherein the first polarizing plate issubjected to a low-reflection treatment.
 3. The image display mirror fora vehicle according to claim 1, further comprising a λ/4 plate on aviewer side of the first polarizing plate.
 4. The image display mirrorfor a vehicle according to claim 1, wherein: the image display apparatuscomprises a liquid crystal display apparatus including a liquid crystalcell; and the liquid crystal display apparatus is free of a polarizingplate on a viewer side of the liquid crystal cell.
 5. A method by whicha driver of a vehicle observes surroundings of the vehicle with theimage display mirror for a vehicle of claim 1, the method comprising:switching attached and removed states of the first polarizing plate whenan image is displayed on the image display apparatus and when the imageis not displayed thereon; and arranging, when the image is displayed,the first polarizing plate between the half mirror and the driver of thevehicle.